Method of controlling the concentration of impurities in semi-conducting materials



June 21, 1955 .J. ROTHSTEIN 2,711,379

METHOD OF CONTROLLING THE CONCENTRATION OF IMPURITIES IN SEMI-CONDUCTINGMATERIALS Filed Aug. 4, 1952 FIG. 2

INVENTOR.

JEROME ROTHS'TEIN ATTORNEY METHOD OF CONTROLLING THE CONCENTRA TION OFIMPURITIES IN SEMI-CONDUCTING MATERIALS Jerome Rothstein, Belmar, N. J.,assignor to the United States of America as represented by the Secretaryof the Army Application August 4, 1952, Serial No. 302,639

3 Claims. (Cl. 1481.5) (Granted under Title 35, U. s. Code 1952 see. 266The invention described herein may bemanutactured and used by or for,the Government for governmental purposes without the paymentof anyroyalty thereon.

This invention relates to methods of controlling the ice" 4 theelectrodes and 5 into the material to be purified. A

potential difference between the electrodes must be main-* tained whichis large compared to thermalenergiesat the temperature of operation(measured; in electron volts lflj divided .by the chargeonan electron.lfojur or-five volts I is more than ample. The higher potentialdiilerence,

the greaterthe speed and efiicacy of electrolysis; The

" electrolysis is continued until the ions ,have been sweptconcentration of impurities in semi-conducting materials such asgermanium, silicon, and other materials.

Purification of germanium, silicon and other high resistivity materialsto the degree needed in semi-conductive work is very difiicult toattain. Even traces undetectable chemically may have decisive effects onthe electrical properties of semi-conductors. Heretofore the approachhas been essentially that of chemical purification followed bycrystallization techniques.

The present invention contemplates purification of A semi-conductingmaterial by electrolyzing the materials whereby donor type impurities,which bear an effectively positive charge, will migrate toward thecathode, while acceptor type impurities, which bear an efiectivelynegative charge will migrate toward the anode. N-type ma terial willthus form around the anode, P-type around A the cathode, withexceptionally pure material in between.

The invention and its objects will be illustrated by the followingdescription of specific embodiments taken in connection with theaccompanying drawing wherein Fig.

1 shows a top view of a crucible for-melting the get-' maniumwith theheating coils shown in horizontal sectional view and Fig. 2 a verticalsectional viewv of the assembled apparatus.

Germanium is heated to about its melting point which is 959 C. It isdesirable to approach or exceed themelting point so as to get suflicientmobility of the'impurity ions. Exceeding the melting point isadvantageous in into the enlarged regions 2'and'3'which contain'theelec- A, trodesleaving a very small concentration of impurities in' theregion in between.

The high temperature needed to the materialmayf befobtained by radiofrequency heating or bythe'use of conventional heating coils'S and 9surrounding thecru:

cible 1 why other conventional arrangements.

After finishing :the electrolysis, in the event that material has beenmelted, cooling is started by first cooling the central region of themelt whichcontainsithe pure material ,beforecooling the partsnear therelectr'odes where theimpurities gather. Otherwise the crystalliza- Htion forces would push the impurities back into the puri= fied region, 1This selective coolingmaybe accomplished in variousways for instance bymaking the heating c'o ils 1 8 and 9 slidable inahorizontal direction(not sho'wnzi'n the dr'aWin gyand by pulling these coils with-the aidof,

rods 10 andll'toward the electrodes after the electrolysis V isfinished.

To prevent oxidation of the heated materialthe elec-, trolysis may becarried out in vacuo or in an inert atmosphere. For this purpose thecrucible 1 is inserted into 1/ f tube 14 of an appropriate material,such asquart; glass. The water-cooled" holders 6 and 7 and the wiring@12 and V t 13 for the heatingcoiIs-S and 9 are hermeticallysealed I tinto fthewallsof thetube '14. ,The ends otjtube 14 (not i shown in thedrawings) .iare closed and" connected in well} known rnanner withthenecessary implements forsupply- I ing either high vacnumor an inertatmosphere;- Y Away of circumventing contamination of moltenlsemiconductingmaterial by the containe'r'lis tor-the molten 1 I portion tobe surrounded by a solid cocled outer porltion whereby the outer coolerand solid portion of the ma terial (such as silicon or germanium) act asthe container that themobility is greater in the liquid state than inthe A solid. However, the disadvantage of going to very hightemperatures is that impurities begin to diffuse into the melt from thecontainer. I

The use of electrolysis for the purpose of controlling the concentrationof impurities is not obvious because semi-conductors conductelectronically, rather than ionia cally. Even under the conditions thatexist in the above mentioned melt most of the current may be carried byelectrons or holes (or both). At the temperatures con templated,however, the lattice structure of the solid is broken down either bymelting or by generation of lattice defects below the melting point tosuch a high degree that atoms and ions become quite mobile whereby thecharged impurities can drift under the influence of an i appliedelectric field. i

The apparatus shown in Figs, 1 and 2 of the drawing uses an elongatedshallow quartz crucible 1' having enlarged and deepened ends 2 and 3into which pure carbon electrodes 4 and 5 are dipped. The germanium inthe immediate vicinity of the electrodes is cool and solid because ofthermal loss thru the inserted carbon electrodes.

The carbon electrodes 4 and 5 are preferably held by water-cooledholders 6 and 7 to insure cooling of the parts of the metal in theneighborhood of these carbon electrodes. This prevents diffusion ofimpurities from untilino further changeoccursone can obtain regions'ofN-type and P-type material around the electrodes and a pure material inthefcentral region)": If," however, electrolysis is stopped at anintermediate point rnaterials with, variousdistributionsof donor andacceptor type impurities h and materials. with .various' degrees ofpurity will :bef obtained; 7 7; y Aparticular advantage of the present'invention overthe known purification by crystal growing methods cone i-.sists in the fact that very large batches of semi-conductingmaterialmay be purified. The amountsto be purified with the aid ofelectrolysis are limited only by the power available.

What isclaimed-is:

'1; Method of controlling the concentrationof impuri tiesinsemi-conducting materials comprising submitting a material selectedfrom the group consisting 'of'germariitim and silicon to the action ofan electric field in" the presence of spaced anode and cathodeatatemperature that brings; 7

about sufiicient "mobility ,offthe' impurity ions whereby I donor type.impurities migrateioward thecathcde, While acceptor type impuritiesmigrate toward the anode, coir tinuing' the treatment until material ofthe desired degrees I of purity and impurity is obtained, andselectively cooling the-material byfirst coolingthe central-region thatcon- Patented .lune .21, 1955:

3 tains the desired degree of purity before cooling the parts near theelectrodes.

2. Method of controlling the concentration of impurities insemi-conducting materials according to claim 1 in which the material isheated in an inert atmosphere to about its melting point.

3. Method of controlling the concentration of impurities insemi-conducting materials according to claim I in which an inner portionof the material is heated whereby the outer cooler portion of saidmaterial acts as the container 'for the inner molten portion.

References Cited in the file of this patent UNITED STATES PATENTS

1. METHOD OF CONTROLLING THE CONCENTRATION OF IMPURITIES INSEMI-CONDUCING MATERIALS COMPRISING SUBMITTING A MATERIAL SELECTED FROMTHE GROUP CONSISTING OF GERMANIUM AND SILICON TO THE ACTION OF ANELECTRIC FIELD IN THE PRESENCE OF SPACED ANODE AND CATHODE AT ATEMPERATURE THAT BRINGS ABOUT SUFFICIENT MOBILITY OF THE IMPURITY IONSWHEREBY DONOR TYPE IMPURITIES MIGRATE TOWARD THE CATHODE, WHILE ACCEPTORTYPE IMPURITIES MIGRATE TOWARD THE ANODE, CONTINUING THE TREATMENT UNTILMATERIAL OF THE DESIRED DEGREES OF PURITY AND IMPURITY IS OBTAINED, ANDSELECTIVELY COOLING THE MATERIAL BY FIRST COOLING THE CENTRAL REGIONTHAT CONTAINS THE DESIRED DEGREE OF PURITY BEFORE COOLING THE PARTS NEARTHE ELECTRODES.